A preview of some of the most interesting papers appearing in the February 2008 issue of the Journal of the SID. To obtain access to these articles on-line, please go to www.sid.org

Edited by Aris Silzars

PIN OLEDs – Improved structures and materials to enhance device lifetime

Jan Birnstock
Tobias Canzler
Michael Hofmann
Andrea Lux
Sven Murano
Philipp Wellmann
Ansgar Werner

Novaled AG

Abstract — Currently, three issues are identified that decide upon the commercial success of organic light-emitting diodes (OLEDs), both in display and lighting applications: power efficiency, lifetime, and price competitiveness. PIN OLEDs are widely seen as the preferred way to maximize power efficiency. Here, it is reported that this concept also delivers the world's longest lifetimes. For a highly efficient deep-red PIN OLED, a half-lifetime of 25,000 hours for a starting brightness of 10,000 cd/m2 and a minimal voltage increase over lifetime is reported. This value corresponds to more than 1 x 106 hours at 1000 cd/m2 using an exponent of n = 1.7, which was measured by driving the OLEDs at different starting luminances. Because there is no initial luminance drop, these PIN OLEDs also exhibit a very high 80% lifetime (>300,000 hours at 1000 cd/m2). New record lifetime values for blue and green will be reported as well. Additionally, further topics that have impact on the production yield and cost such as the newly developed air-stable organic n-doping material NDN-26 and top-emitting structures will be discussed.

The origins of redox-doped OLEDs, so-called PIN OLEDs, date back to the nineties. At that time, it was believed that devices with low driving voltage and therefore high power efficiency could be achieved with this approach. However, it took about one decade until this promise could be accomplished – in 2004, a power efficiency of 64 lm/W for a green-phosphorescent PIN OLED was published. The next breakthrough in the development of PIN OLEDs was certainly the increase of the half-lifetime above 100,000 hours at an initial brightness of 500 cd/m2.



FIGURE 13 — Lifetime measurement of a deep-red-phosphorescent PIN OLED (top emitting) comprising the n-dopant NDN-26 and the emitter system TMM-004:TER-004. The OLED features a current efficiency of 31 cd/A and a driving voltage of 2.7 V at 1000 cd/m2.


Development of a phosphorescent white OLED with extremely high power efficiency and long lifetime

Tomoyuki Nakayama
Kunimasa Hiyama
Keiichi Furukawa
Hirofumi Ohtani

Konica Minolta TechnologyCenter, Inc.

Abstract — A white OLED device with extremely high power efficiency and long lifetime was developed, in which blue, yellow-green, and red phosphorescent emitters were used. The performances achieved were 64 lm/W and 10,000 hours of lifetime at an initial luminance of 1000 cd/m2 by using a light outcoupling technique. The device also exhibited the good durability important for practical usage. New technologies, such as blue phosphorescent materials and a sophisticated organic layer structure, were applied to the device. Hopefully, these technologies will open the door to the practical use of OLEDs as light sources.

As shown in Fig. 11, the performance of our OLED device surpassed that of electric bulbs and reached the domain of fluorescent lamps. Although further study will be needed in the future, encouraging durability characteristics, such as the storage stability essential to practical usage, were obtained.



FIGURE 11 — Light-source trends; combined power efficiency and lifetime.


Novel highly reflective and bistable electrowetting displays

Karlheinz Blankenbach
Andreas Schmoll
Andriy Bitman
Frank Bartels
Dieter Jerosch

Pforzheim University

Abstract — Novel displays have been developed using the effect of moving a droplet by electrowetting. This approach enables bistable and reflective monochrome and color displays which could also be made on plastic substrates. Prototypes show promising performance in terms of contrast ratio, gray scale, and color.



FIGURE 1 — Electrowetting principle: A water droplet on a hydrophobic layer is contracted without voltage (a) and relaxed by applying an appropriate voltage (b).



FIGURE 14 — CMY stacked EW droplet-driven color prototype displaying several primaries by 2-mm droplets in pure reflective mode.


Recent progress in flexible color reflective cholesteric displays

Asad Khan, Tod Schneider
Erica Montbach, Donald J. Davis
Nick Miller, Duane Marhefka
Todd Ernst, J. William Doane

Kent Displays, Inc.

Abstract — Highly flexible layered full-color cholesteric displays fabricated using ultra-thin substrates with encapsulation through the phase-separation approach is reported. Recent progress of the state of the art of cholesteric display technology will be discussed as well.

The display photographs are shown in Fig. 7. The display is only about 70 μm in total thickness (including the back absorbing layer) and a high degree of flexibility is clearly seen. There is no observable parallax because the display layers are very close to each other. In addition, the contrast, color saturation and reflectivity are high as well. The conducting-polymer coatings have a sheet resistance of about 1 kΩ/4 and a transmission greater than 90%. The absorption in the conducting-polymer layers adds quickly to reduce reflections from the lower layers of the display. However, as evidenced from the photographs, all display layers and colors appear bright and saturated.



FIGURE 7 — Photographs of the full-color stacked ChLCDs using ultra-thin substrates and the phase-separation approach. The display format is 20 x 24 pixels.


High-performance OCB-mode field-sequential-color LCD

Takahiro Ishinabe
Kazuhiro Wako
Kazuo Sekiya
Tadashi Kishimoto
Tetsuya Miyashita
Tatsuo Uchida

Tokoku University

Abstract — Optically compensated bend (OCB) mode is a promising technology for future high-quality display devices due to its wide viewing angle without gray-scale inversion and color shift, fast response time, high contrast ratio, and wide temperature range. This paper summarizes the developments of the OCB mode and the optical performance of OCB-mode field-sequential-color LCDs.

A 15-in.-diagonal OCB-mode FSC TFT-LCD by using a scanning-LED backlight was developed. This scanning-LED backlight is used to realize RGB color fields with high brightness and good uniformity and has 10 blocks with partitioned walls to suppress light penetration from the neighboring blocks and undesirable inter-field color mixture. This is the world's first OCB-mode LCD that operates at 360 fields/sec, and it was confirmed that wide-viewing-angle, high-contrast-ratio, wide-color-gamut, high-quality moving images without color breakup can be obtained.



FIGURE 14 — Prototype 15-in. field-sequential-color OCB-mode LCD.


Viewing-angle compensation of TN- and ECB-LCD modes by using a rod-like liquid-crystalline polymer film

Tetsuya Uesaka
Satoru Ikeda
Suzushi Nishimura
Hitoshi Mazaki

Nippon Oil Corp.

Abstract — A liquid-crystalline retardation-film technology by using a rod-like liquid-crystalline polymer (LCP) for various LCD modes have been developed. In particular, considerable improvements in viewing-angle performance have been achieved for the twisted-nematic (TN) and the transmissive/transflective electrically controlled birefringence (ECB) modes by using hybrid aligned nematic film (NH Film).

As shown in Fig. 9, it was confirmed that the transmissive ECB-LCD with new NH film can display vivid color and clear images in both the horizontal and vertical directions. Although the viewing-angle range (CR > 10) of the ECB-LCD (I) exceeds 160° in the diagonal directions, it reaches only 120° in the horizontal and vertical directions due to the light leakage from the crossed polarizer in the off-axis. This problem can be solved by inserting the uniaxial film 2 into the ECB-LCD (I).



FIGURE 9 — Comparison of image qualities of a TN- and ECB-LCD (I) with NH film at oblique angles.


Oxide TFT with multilayer gate insulator for backplane of AMOLED device

Ho-Nyun Lee
Jaewoo Kyung
Myeon-Chang Sung
Do Youl Kim
Sun Kil Kang
Seong-Joong Kim
Chang Nam Kim
Hong-Gyu Kim
Sung-Tae Kim

LG Electronics

Abstract — An indium gallium zinc oxide (IGZO) film with an amorphous phase was deposited and had a very flat morphology with a RMS value of 0.35 nm. IGZO TFTs were fabricated on a glass substrate by conventional photolithography and wet-etching processes. IGZO TFTs demonstrated a high mobility of 124 cm2/V-sec, a high on/off ratio of over 108, a desirable threshold voltage of 0.7 V, and a sub-threshold swing of 0.43 V/decade. High mobility partially resulted from the fringing-electric-field effect that leads to an additionalcurrent flow beyond the device edges. Therefore, considering our device geometry, the actual mobility was about 100 cm2/V-sec and had a very low dependence on the variation of W/L (channel width and length) and thickness of the active layer. IGZO TFTs were also fabricated on a flexible metal substrate for a conformable display application. TFT devices showed an actual mobility of 72 cm2/V-sec, a high on/off ratio of ~107, and a sub-threshold swing of 0.36 V/decade. There was no significant difference before, during, or after bending. Moreover, an IGZO TFT array was fabricated and a top-emitting OLED device was successfully driven by it. Therefore, the oxide TFT could be a promising candidate as a backplane for OLED devices.

Figure 7 shows as-fabricated IGZO TFTs on a stainless-steel foil. 100-μm-thick stainless foils have sufficient flexibility to be bent into a curve with the radius (R) of 40 mm. IGZO TFTs were fabricated with a 50-μm channel width (W) and 20-μm channel length (L). Figure 8(a) is the transfer characteristic curve of the IGZO TFT. The IGZO TFT showed a high drain current (IDS) of 120 μA for the conditions of VGS = 10 V and VDS = 10 V. Moreover, the TFT showed a low off-current of about 2 pA, which resulted in a high on/off ratio of over 107.



FIGURE 7 — IGZO TFTs fabricated on stainless-steel foil.


Practical CNT-FED structure for high-luminance character displays

Junko Yotani
Sashiro Uemura
Takeshi Nagasako
Hiroyuki Kurachi
Tomotaka Ezaki
Tsuyoshi Maesoba
Takehiro Nakao
Masaaki Ito
Akira Sakurai
Hideo Shimoda
Hiromu Yamada
Yahachi Saito

Noritake Co., Ltd.

Abstract — A high-luminance CNT-FED character display using a simple line-rib structure was constructed. The display panel had 48 x 480 dots and the subpixel pitch was 1 mm. The greatest benefit of a display using CNT technology is high luminance performance with low-power consumption. The luminance of the green-color dot was ca. 10,000 cd/m2 under 1/16 duty-cycle driving at a 6-kV anode voltage. The high luminance of the display panel can provide good visibility when installed even in outdoor locations, and the power consumption was ca. 4 W at the character displaying module. Thus, a CNT-FED for character displays also has potential multifunctionality, which could be battery driven. It should be useful for public displays even under emergency no-power conditions. In this work, a practical structure and process technologies for making ribs with reasonable cost were developed. The newly introduced 2-mm-tall line ribs as spacers were formed by using innovative production processes; i.e., the rib paste was pushed out of a multi-slit nozzle, and the rib shape was formed by UV-light irradiation. The developed panel structure and manufacturing processes also had the advantages of size flexibility and high production yield.



FIGURE 4 — Schematic of the structure of a practical new panel using line rib spacers.



FIGURE 11 — Photographs of a displayed color character pattern. The display area is 480 mm x 48 mm. (a) A prototype device. (b) A photograph of a battery-driven demo display.


Progress of LED backlights for LCDs

Munisamy Anandan

Organic Lighting Technologies LLC

Abstract — Cold-cathode fluorescent lamps (CCFLs) are being used for LCD backlighting and is currently the dominant technology for LCD backlighting. However, recent attention has been given to LEDs as light sources for LCD backlighting because of their (i) long life, (ii) low-voltage operation, (iii) fast response time, and (iv) wide color gamut. This review article commences with the basics of LEDs as light sources and their limitations, followed by various backlight structures employing LEDs in cell phones, notebook computers, and LCD TVs. The description of the improvement in image quality on an LCD screen, stemming from the characteristics of LEDs, is also given. In conclusion, the possible rapid growth of LED backlights is outlined, thus gradually ending the domination of CCFL backlights.

The performance level of LED backlights has exceeded that of CCFL backlights and is providing a boost to LCDs in enhancing its image quality and decreasing its power consumption. The barrier for penetration is their cost. For white LED backlights, the cost is not as large a barrier as the RGB-based white-LED backlight. With the substantial advantages consumers are getting with white LED backlights vs. CCFL backlights for laptop computers and the low-power (20 mA) white LED selling at low prices, white LED-backlight market for NBCs will significantly increase. A 15% market penetration can happen in 3 years with the unit requirement by the year 2010 reaching nearly 120 million units.



FIGURE 41 — RGB color control system employing LED backlights.


Power savings and enhancement of gray-scale capability of LCD TVs with an adaptive dimming technique

Tomokazu Shiga
Sho Shimizukawa
Shigeo Mikoshiba

University of Electro-Communications

Abstract — The luminance of a backlight unit for an LCD TV is adaptively and locally dimmed along with the input video signal in order to reduce the power consumption and also to improve the picture quality. By adopting the zero-dimensional (0D), 1D, and 2D adaptive dimming techniques, a sample movie having 8.0% post-gamma average picture levels (APL) could be displayed using 83%, 71%, and 50% of the original backlight power, respectively. For an adoption of the 2D dimming, an LED backlight is preferable. The adaptive-dimming technique also allows the differential aging characteristics between the LED components and temperature dependence of color and luminance to be overcome. From simulations of a reduction in power consumption, it was found that 40 x 40 pixels is a unit of the local dimming, 30 frames for the sampling period, 24 dimming steps, and an equal-signal-step method for determining the dimming factor have been found to be appropriate. The gray-scale capability of low-luminance images can also be improved by dimming the backlight luminance and expanding the input signal. By using an LCD TV having an 8-bit capability, an 11-bit-equivalent gray-scale expression was experimentally proven.

Figure 5 shows a structure of an experimental LED backlight unit used for the 2D-dimming investigation. The unit is divided into 3 x 4 blocks by using 0.2-mm-thick white optical isolators. A 19-in.-diagonal IPS-mode SXGA LCD module having a pixel pitch of 0.294 mm is mounted on top of the backlight unit. Each block corresponds to 102 x 102 LCD pixels. Due to the small size of the experimental backlight unit, only a portion of the LCD module is employed. The dimming experiments were performed using two 10-sec sample movies, consisting of 306 pixels vertically and 408 pixels horizontally.



FIGURE 5 — LED backlight unit for 2D adaptive dimming.


Design and fabrication of a micropatterned polydimethylsiloxane (PDMS) light-guide plate for sheet-less LCD backlight unit

Joo-Hyung Lee
Hong-Seok Lee
Byung-Kee Lee
Won-Seok Choi
Hwan-Young Choi
Jun-Bo Yoon

Abstract — A polydimethylsiloxane (PDMS) light-guide plate (LGP) having micropatterns with an inverse-trapezoidal cross section was developed for a sheet-less LCD backlight unit (BLU). The micropatterned PDMS LGP was fabricated by back-side 3-D diffuser lithography followed by two consecutive PDMS replication processes: photoresist-to-PDMS and PDMS-to-PDMS replications. The fabricated LGP showed an average luminance of 2878 nits and a uniformity of 73.3% in a 2-in. backlight module with four side-view 0.85-cd LEDs. It also could feasibly be applied to a light source for flexible displays owing to the flexible characteristic of the PDMS itself.

Compared with a conventional LCD BLU, which has three optical sheets and one reflector film, the proposed BLU is comprised of only a PDMS LGP having micropatterns with an inverse-trapezoidal cross section on the front surface to emit light upward from the side-view LEDs as shown in Fig. 1. In this work, the optical properties of the fabricated LGP such as luminance profile and angular distribution when mounted in a 2-in. backlight module are described.



FIGURE 1 — A schematic view of the proposed micropatterned PDMS LGP for sheet-less LCD BLU.


A novel diffractive backlight concept for mobile displays

Jyrki Kimmel
Tapani Levola
Pasi Saarikko
Johan Bergquist

Nokia Research Center

Abstract — Power-efficiency demands on mobile communications device displays have become severe with the emergence of full-video-capable cellular phones and mobile telephony services such as third-generation (3G) networks. The display is the main culprit for power consumption in the mobile-phone user interface and the backlight unit (BLU) of commonly used active-matrix liquid-crystal displays (AMLCDs) is the main power drain in the display. One way of reducing the power dissipation of a mobile liquid-crystal display is to efficiently distribute and outcouple the light available in the backlight unit to direct the primary wavelength bands in a spectrum-specific fashion through the respective color subpixels. This paper describes a diffractive-optics approach for a novel backlight unit to realize this goal. A model grating structure was fabricated and the distribution of outcoupled light was studied. The results verify that the new BLU concept based on an array of spectrum-specific gratings is feasible.

Figure 1 shows a schematic outline of the new pixelated diffractive backlight concept. Incoming light is fanned out by a grating that is selective for each primary color, and respective red, green, and blue LEDs are used for the color primaries. The light is then distributed throughout the active area of the display by total internal reflection (TIR). In the active area, an array of gratings couples out the light into the active aperture of the display pixel matrix. The separation of color primaries at the pixel level is achieved by color-specific gratings, and by orienting the green-primary light propagation inside the TIR light guide perpendicularly to the red and blue primaries.



FIGURE 1 — Pixelated backlight concept (not to scale).


A YC-separation-type projector: High dynamic range with double modulation

Yuichi Kusakabe
Masaru Kanazawa
Yuji Nojiri
Masato Furuya
Makato Yoshimura

NHK Science & TechnicalResearch Laboratories

Abstract — An experimental projector that features double modulation to obtain high-resolution (4096 x 2160 pixels) and high-dynamic-range images has been developed. Although a conventional projector contains three modulators for red, green, and blue and output light after combining the modulated light from these three sources, our projector has an additional modulator for luminance that modulates the combined RGB modulated light. It can display high-resolution color images by combining three low-resolution panels for chrominance modulation and one high-resolution panel for luminance modulation. In addition, the dynamic range is dramatically improved because the double-modulation scheme minimizes blacklevels in projected images. The projector demonstrates an extremely high dynamic range of 1.1 million to 1- and 10-bit tone reproduction.

Figure 9 shows examples of parts of projected images on each stage; the image modulated only by RGB signals, the image modulated only by a Y signal, and the total image with double modulation. The image modulated only by RGB signals appears blurred because of the low resolution of the RGB signals and relay lens. However, the resolution of the output image with double modulation is high. These photographs prove that a high-resolution color image is obtained only if the resolution of the Y signal is high.



FIGURE 9 — Examples of parts of projected images. Upper left – image modulated only by RGB signals (low resolution). Upper right – image modulated only by a Y signal (high resolution). Lower – output image with double modulation.